Nowadays it is of common practice in dentistry the use of polymer coatings, for both sealant preventive purposes of pit and fissure (adhesives, etc.) and restorative (sealants), such as aesthetic purposes. In most of these materials, the polymerization (curing) process starts via a free radical mechanism, produced both chemically (self-curing systems) or by the action of light (light-cured systems). Despite progress in the development of new polymeric dental materials, there have been some problems in their application; including chemical degradation, mechanical damage of the polymer in the environment of the oral mucosa and allergic and toxicological problems.
Most photo-polymerizable dental coatings containing a mixture, carefully selected from mono- and multifunctional acrylic monomers, being one of the most used during the past years, 2-bis[p-(2-hydroxy-3-metacryloxypropoxy)phenyl]propane, commonly known as bisphenol A-glycidyl methacrylate (BisGMA), which was developed in 1956 by Rafael Bowen1.
The BisGMA is mutagenic and carcinogenic, its chemical structure has two reactive links at both ends of the molecule. Studies show that DNA synthesis is affected in the presence of BisGMA at concentrations of 5 μmol per liter, in addition to exhibiting estrogenic actions. In cells, after contact with adhesives, as eventually occur during removal procedures of the appliances. There are also reports of allergic dermatitis in the dental field staff that has been attributed to the release of monomers of dental resins (composites)2.
Bisphenol A plays an important role in almost all composites and sealants, and is the integral part of the molecule of BisGMA; Bisphenol A found in the mouth may originate from dental materials either to be used as direct ingredient, as a degradation product of other ingredients, such is the case of using bisphenol A dimethacrylate (BisDMA) (because this is exposed in oral cavity to the degradation of the esterases of the saliva); Bisphenol A found in the mouth may be caused to be present as trace material derived from the manufacture of other ingredients, such is the case of products containing BisGMA on sealants and adhesives compounds, because Bisphenol A is the precursor of BisGMA, so that residues may be present in the final product, this is very common if the reactions during manufacture are not adequately controlled.
In a 1996 study, detectable levels of Bisphenol A were reported in the saliva of patients treated with dental sealants (Nicolas Olea, et al. 1996)4, in other studies (Fung, et al, 2000)5 detectable levels are shown to be below reported levels by Olea. The short-term administration of BisGMA and/or bisphenol A in animals or cell cultures can induce changes in estrogen-sensitive organs or cells, however, considering the doses and routes of administration could conclude that the estrogenic effects when using dental composites containing or based on BisGMA is insignificant, and some studies have shown that Bisphenol A is not hydrolysed by BisGMA and have not found significant amounts of BISPHENOL A in dental sealants or composites; Notwithstanding the foregoing, studies are needed to evaluate the long-term effects, taking into consideration that the orthodontist are systematically exposed to these compounds and that users of orthodontic appliances (braces, bonded lingual retainers, etc.) maintain contact with these materials for an average of two years, the duration of treatment. The current trend is to minimize human exposure to Bisphenol A in dental materials as a health precaution6,7.
Furthermore, the BisGMA is a monomer having a high molecular weight, and by their stereotactic structure and high viscosity, presents a phenomenon of steric hindrance during the polymerization, and resulted in a rigid and inflexible material obtained to move the space, this makes it difficult to handle, so that it should add another monomer such as triethylene glycol dimethacrylate (TEGDMA) to dilute its hard consistency and provide better performance.
With respect to the types of adhesives developed for improving the performance or facilitate some of the orthodontic procedures, they can be found in the following patent documents:
The U.S. Pat. No. 6,090,867 relates to a dental adhesive system that allows easy removal of ceramic braces from the adhesive enamel surface based on acrylate comprising a plasticizer which gives ductility, and wherein the plasticizer has poor solubility in water or saliva, it does not evaporate at the temperature of the oral cavity and is not toxic at the doses used.
The U.S. Pat. No. 5,575,645 relates to an orthodontic appliance placed on a container and contains a stable adhesive to be stored, comprising EBDA and is free of TEGDMA. The EBDA may contain EBDMA, and after polymerize has a bond strength of 25 to 54 kg/cm2 has a consistency of 12 to 24.5 mm.
The U.S. Pat. No. 5,295,824 relates to a plastic brace (polycarbonate) with a first layer (monomeric acrylic adhesive) adhered to the surface that makes contact with the tooth and its function is to increase the bond strength between the brace and the dental adhesive; to obtain this type of auto-adhesive brace, it is required applied to the brace a solvating mixture and heated to a temperature of 60 to 100° C. to volatilize the solvents, and that the monomers and with the solvation mixture conform to the primer, and requires 2-30 minutes to diffuse to the surface of the brace that will make contact with the tooth, it can only ensure the stability of the brace during the first few weeks, so this method besides being laborious inefficient.
The U.S. Pat. No. 5,810,584 refers to a brace that carries the pre-applied adhesive on the bonding surface, and is characterized by not being sticky, it mentions formulations of these adhesives, all of these adhesives are based on BisGMA.
Adobes-Martin has reported the effectiveness of self-etching adhesives in the cementing braces. The self-etching adhesives are an alternative for orthodontic cemented, these substances are a weak acid incorporated to the primer, that is why they call it of “one-step”, and the number of procedures is reduced. These products combine the conditioner of the enamel along with the hydrophilic adhesive system in a single vial, allowing simplify the first 2 steps of the braces cementation (etching with orthophosphoric acid followed by the application of bonding resin) and transforming them into a single gesture, the bond strength of these adhesives should be optimal, but no results are provided nor other data reported of parameters expected for a good performance in other aspects of its functions9.
There is another procedure where etchant acid is rubbed and must be washed and dried; then it applied to a primer, which is photopolymerized for 5 to 10 seconds depending on the manufacturer; after the adhesive is applied to the brace, it is positioned on the enamel and polymerized for 10 to 15 seconds. With our experimental one-step adhesive, the step that is removed is the first application; i.e., the etchant acid is rubbed over the tooth (phosphoric acid 37%), washed and dried; the adhesive is placed on the brace and this is placed on the tooth and polimerized for 10 seconds. We are eliminating a polymer layer (the primer) to which is going to adhere the polymer that was placed in the brace, so that the adhesive will polimerized within retentions created in the enamel surface by the acid etching.
The thesis entitled “Valoración física de una resina compuesta a base de Trimetilolpropano trimetacrilato” (Student: Montiel Millan, December 2010 Tutors: Sáez Espínola G. López Palacios E.), aimed to evaluate the physical performance of an experimental resin for reconstruction, based on Trimethylacrilate Trimethylolpropane; however, by the results in the conclusions it is stated that a polymer with adequate performance for the reconstruction of teeth subjected to masticatory loads are not obtained, it was concluded that the silver formulation did not have the desired performance, so changes were needed and again experimental evaluations.
The work presented in “Congreso intede la IADR división mexicana” held in May 2013 entitled “Effect of bi, tri and penta functional monomers on bond strength of denture base resin to acrylic teeth” (Student: Reséndiz Melgar H., Tutor Barceló Santana, F: H. Advisor Álvarez Gayosso C.) aimed to evaluate the stress of base denture adhesion of PMMA (polymethylmethacrylate) and experimental adhesives prepared with monomers bi, tri or multifunctional. It was concluded that resistance is comparable with commercial adhesives of the denture-teeth type of PMMA.
By consequence of this, it has been sought to omit the drawbacks of adhesives, primers and systems of the prior art, by developing a BisGMA-free orthodontic adhesive that is used in one-step and provides a system that is formed by a primer and slurry. The use of adhesives of the present invention is easy handling and further shows adhesion to both the base of the attachment and the enamel.